An RFID tag (which is also called a radio tag or a responsor, or the like) includes an antenna, a radio communication unit, and a storage unit. The storage unit of each RFID tag stores identification information which does not overlap each other between RFID tags. An interrogator (which is also called reader and writer) transmits an inquiry signal to the RFID tag, the RFID tag performs a process in response to the inquiry signal. For example, there is known an RFID tag that replies only when the identification information stored by the RFID tag and the identification information included in the inquiry signal from the reader and writer are identical with each other.
The RFID tag having such a function is used in various fields such as logistics for article management. In recent years, for example, there has been introduced a system for sales or stock management using the RFID tags at stores selling various articles.
Business treating articles in large quantity desires to easily recognize the places where the respective articles are located as well as management of the kinds of articles or the number of articles. In order to detect the positions of the RFID tags, it is possible to use a method of calculating the absolute positions of the RFID tags by performing a process of acquiring the absolute positions at which the reader and writer communicate with the RFID tags and the distances with the RFID tags at least at three locations.
However, when the above method is used, the reader and writer have to acquire the position of the reader and writer and the distances with the RFID tags at least at three locations. Therefore, it takes relatively long time to acquire the positions of the RFID tags. Further, the radio wave use efficiency becomes deteriorate since the reader and writer have to communicate with the same RFID tag at least three times.
In consideration of this circumstance, it is necessary to devise a method of detecting the positions of the RFID tags rapidly.